It's the smart parts tank system. In this case it acts as the integrated tank for the marker, so if you choose to use a different tank system then you'll need some other parts too.
As told by Stobaeus ...someone who had begun to learn geometry with Euclid, when he had learnt the first theorem, asked Euclid "What shall I get by learning these things?" Euclid called his slave and said "Give him threepence since he must make gain out of what he learns".
